In an ad hoc network system, ad hoc network communication terminals (node devices, or simply referred to as nodes) are autonomously connected to one another over a network to perform communications. The term “autonomously” means no necessity to have a communication terminal or an infrastructure, to be used by a user, dedicated to set a communication route and manage a communication using a server and a router.
As a routing to of an ad hoc network (ad hoc protocol), a reactive AODV (ad hoc on demand distance vector algorithm) examined by the MANET WG of the IETF, and a proactive OLSR (optimized link state routing) are well known.
Also known is a proactive routing protocol for preparing a route for an autonomous data communication by each node prior to a communication request, learning the optimum route at any time, and changing a route (Patent Document 1). In the above-mentioned ad hoc network, each node periodically communicates a message like a hello message including node information such as route information, communication quality information about a link between nodes, etc. between adjacent nodes to know how each node is connected to other nodes. By communicating the above-mentioned message, the optimum route may be learned at any time to change a route.
In an example of a mesh ad hoc network, a node which is connected to a network server (for example, a gateway (GW)) acquires time information from the network server after it is activated, and broadcasts a time synchronous frame to a plurality of nodes subordinate to the upper node, thereby performing time notification. Upon receipt of the time notification, a node sets it as the system time of the local device, and generates a shared key for encryption/decryption based on the time. Therefore, a node whose system time is not set is unable to generate a shared key for decryption although it receives encrypted data from another node. In the above-mentioned ad hoc network, an encryption key which is generated based on the time information is required in communicating data between nodes. Accordingly, each node communicates data when time-synchronization is performed, thereby creating a network.
In a time-synchronized mesh ad hoc network, the transmission period of a time synchronous frame required to acquire time-synchronization between a new node and a node which belongs to an existing network may be, for example, once a day. In this case, a node which is newly entering the network may enter the network only after receiving a time synchronous frame once a day when surrounding nodes periodically transmit the frame, and has to wait for the reception for a day at a maximum after the activation. The transmission period of a time synchronous frame is generally longer than the transmission period of a message including route information such as a hello message and node information such as the communication quality information about a link between nodes.
[Patent Document 1] International Publication Pamphlet No. WO2011/013165